Question

Carbon dioxide is always made of ____ parts of oxygen to ____ parts carbon by mass. It’s asking for the mass ratio of these elements.

Answer 1

First look at your periodic table for the atomic masses of oxygen and carbon.

Carbon: 12 grams
Oxygen: 16 grams

In carbon dioxide there are 12 grams of carbon and 32 (16 x 2) grams of oxygen because Di means two.

The unreduced ratio of oxygen to carbon would be 32:12

Reduce it:
32:12 = 16:6 = 8:3

The ratio is 8:3

Answer 2

Answer: Cabon dioxide is always made of 8 parts of oxygen to 3 parts carbon by mass.

Explanation:

Law of definite proportion states that the elements combining to form compounds always combine in a fixed ratio by their mass.

The chemical formula for carbon dioxide is $CO_2$

In 1 mole of carbon dioxide, 1 mole of carbon atom and 2 moles of oxygen atoms are present.

We know that:

Molar mass of carbon = 12 g/mol

Molar mass of oxygen = 16 g/mol

In carbon dioxide molecule:

Mass of carbon atom = 12 g

Mass of oxygen atom = (2 × 16) = 32 g

Mass ratio of oxygen and carbon in carbon dioxide molecule is:

$\Rightarrow O:C=32:12=8:3$

Hence, carbon dioxide is always made of 8 parts of oxygen to 3 parts carbon by mass.